Ionization detectors for gas chromatography are well known in the art. A comprehensive survey of such detectors as of 1961 may be found in an article entitled "Ionization Methods for the Analysis of Gases and Vapors" by J. E. Lovelock, Analytical Chemistry, Volume 33, No. 2, February 1961, pages 162-178. The detectors reviewed in that article include, inter alia, the cross-section ionization detector, the argon detector, and the electron capture detector. These detectors are characterized by the fact that each includes a source of ionizing radiation, i.e., a radioactive material.
The use of radioactive substances in chromatographic detectors necessarily introduces certain health risks into the laboratory and complicates such tasks as cleaning detectors after use. Because of these health risks, they are also subject to certain governmental controls which complicate their application and use.
Ionization detectors have been developed which avoid the need for radioactive elements. However, in many cases, these are not suitable for use as argon and electron capture detectors for various reasons, among which is the fact that they may require other gasses in addition to the carrier or sample. Examples are the photo-ionization detector referenced in the above-mentioned Lovelock article and the flame ionization detector (FID).
An electron capture detector has been developed which utilizes a filament as a thermionic emission electron source. Such a detector is described in U.S. Pat. No. 4,304,997 of Sullivan et al. However, there are certain problems inherent in such a thermionic detector. One such problem is that the emitting filament is at very high temperature, is of limited area, and may be "poisoned" by components of many samples--i.e., components may be adsorbed on the surface and thereby reduce its thermal emissivity.
In a more recent development, electrons for use in argon and cross-section detectors have been produced by the action of ultraviolet radiation on a suitable metallic surface. Such a detector has functioned satisfactorily but required an ultraviolet source, windows transparent to ultraviolet radiation, and satisfactory electron emitting surfaces.
For the foregoing reasons, it is a primary object of this invention to provide an ionization detector which avoids the need for ionizing or optical radiation, additional gases, heated filaments, or dedicated electron sources. Other objects, features, and advantages will become apparent from the following description and appended claims.